Hybrid vehicles, which are driven by an internal combustion engine and one or more electric machines, or purely electrically driven vehicles can operate the electric machine(s) as a generator during a brake actuation in order to convert the kinetic energy of the vehicle into electric energy and advantageously store said electric energy in a battery.
WO 2011/092308 A1, which is incorporated by reference discloses a brake system for a motor vehicle, which comprises at least one electric regenerative brake and one pressure medium-operated brake system having wheel brakes, wherein the wheel brakes assigned to the individual wheels are disposed in two brake circuits, to which a pressure medium can be supplied from a first braking pressure-generating means, a tandem master brake cylinder, and wherein each brake circuit comprises at least one pressure accumulator and electronically controllable hydraulic valves, and wherein the brake system comprises an electronic control and regulating unit, by means of which, during braking with the electric regenerative brake(s), at least one hydraulic valve is controlled in such a way that pressure medium is diverted into exactly one, in particular predetermined, pressure accumulator at a point in time. A pressure accumulator is therefore filled at the beginning of a brake actuation, in order provide the driver with the usual pedal feel also during a generator operation of the electric machine. As soon as the first pressure accumulator has a specified fill level, pressure medium is preferably diverted into the pressure accumulator of the second brake circuit. The diverted quantity of pressure medium or brake fluid does not contribute to the pressure increase in the wheel brakes of the vehicle wheels and can vary as a function of the particular generator torque that can be applied, the brake activation by the driver, and the brake circuit split.
An induction-based generator loses nearly all its braking effect at low speeds, which is why the missing drag torque of the generator must be compensated for with the aid of the wheel brakes. This transition between the drag torque of the generator and the braking torque of the wheel brakes, which is also referred to as blending, often takes place in that pressure is built up in the wheel brakes via a pump, which is fed from the pressure accumulator, wherein the pumps of the two brake circuits are usually operated by a common motor. Depending on the brake circuit split, the generator torque that is available at any instant, and the brake activation by the driver, it is possible that the two low-pressure accumulators may not contain an equivalent amount of brake fluid, for reasons related to the system. As a result, while the pump still delivers in one brake circuit, the deliverable volume has been used up in the other brake circuit and a negative pressure sets in, in which vapor pockets can also form. According to the cited prior art, a resultant negative pressure is relieved by temporarily opening the electronic switching valve disposed between the pressure accumulator and the master brake cylinder as soon as the brake actuation has ended; this is also referred to as an electronic switching valve degassing pulse.
A corresponding blending also occurs in the case of a gentle brake activation, in which the vehicle is braked to a standstill with slow deceleration. As a result of the gentle brake activation, the low-pressure accumulators are only partially filled, and even if the pump is activated at a low speed, a negative pressure would be present in a brake circuit, on the suction side of the hydraulic pump, for a relatively long time, which can have detrimental effects on the bleeding state of the brake system.